fix ActorSelection hashCode/equals and ask, see #3287 #3261
remove NullMessage, see #3281
call executor.prepare() for scheduled jobs, see #3307
Merge branch 'wip-3281-NullMessage-∂π'
Merge pull request #1400 from akka/wip-3307-prepare-EC-∂π

call executor.prepare() for scheduled jobs, see #3307
remove ActorRef#tell(Any), see #3293

also remove FakeActorRef from SerCompSpec
Merge pull request #1389 from akka/wip-3287-actorSelection-∂π

ActorSelection: equals/hashCode and “ask”
Merge pull request #1396 from akka/wip-3293-remove-single-arg-tell-∂π

remove ActorRef#tell(Any), see #3293